The invention relates to a method of forming via holes in an insulation film including organic polymers to be used as a substrate for high density multilevel interconnections as well as a method of cutting the insulation film in various desired patterns.
The multilevel interconnection techniques may permit an improvement of a high density packaging of large scale integrated circuits for computers. One of the important issues regarding integrated circuit packaging techniques is to reduce the manufacturing cost as well as to place the high density packaging technique in a practical use. The insulation film used for the high density packaging of the integrated circuits are required to be made of such materials as to permit the integrated circuits to show high frequency performances. TEFLON.RTM. resin, polyimide synthetic resin polymer, and glass epoxy resin are available for the insulation film material wherein the insulation film material may include silica.
In the prior art, the via holes are formed in a glass epoxy resin substrate by use of mechanical drills. Further it has been known in the art that to form the via holes in the organic film photo-etching of polymer films by far-ultraviolet excimer laser radiation is available, which is disclosed in Applied Physics Letter 41(6) 15, Sep. 1982, pp. 576-578. In the prior art for cutting the glass epoxy resin substrate, is has been known to use various mechanical cutter. To cut the organic films, it has been known to use carbon dioxide gas laser radiations to cause a melting of the organic film and subsequent evaporation thereof.
The above method of forming the via hole by use of the mechanical drill has the disadvantage described below. A time of 0.1 seconds is needed to form a single via hole in the glass epoxy resin substrate. As a result, a formation of 2000 of via holes in the 20 cm-squared substrate requires about 3 minutes. This provides a limitation in throughput thereby resulting in a difficulty in reduction of the manufacturing cost. When forming the via hole in the TEFLON.RTM. synthetic resin polymer film suitable for high frequency performance of the device but having poor thermal and mechanical property, it is required that a rotation speed and a feeding speed of the drill are slow. In use of the mechanical drill, a minimum diameter of the via hole is relatively large for example 200 micrometers.
The above method of use of the photo-etching of polymer films by far-ultraviolet excimer laser radiation also has the disadvantages described below. This method may permit forming a fine via hole having a diameter of a few micrometers. Notwithstanding, in use of this laser radiation, a maximum processing speed is 0.1 micrometers/pulse. When using the commercially available excimer laser of 100 Hz, about 2 seconds are needed to form only a single via hole in a thin insulation film having a thickness of 20 micrometers. Even if one shot laser radiation may form a plurality of via holes, then the processing speed to form the via hole in the 20 cm-squared printed board is 1/5 times of the processing speed in use of the mechanical drill thereby resulting in a difficulty in improvement in the throughput. Further, this laser radiation method is available to only a film that shows a high absorption property of the laser beam. The XeCl excimer laser or KrF excimer laser are not available to the Glass epoxy resin film and the TEFLON.RTM. synthetic resin polymer film. The excimer laser uses halogen gases such as fluorine or chlorine. This results in a high cost of the laser device or a high maintenance cost.
The above conventional method for cutting the film by use of the mechanical cutter also has a disadvantage in that the cutting edge of the film has a flash and a chamber. Further, a cutoff area of 300 micrometers is needed.
The use of the carbon dioxide gas laser to cut the film also has a disadvantage in that a thermal transformation caused by the laser radiation remains at the cutting edge of the film because the cutting edge is subjected to a higher temperature than a melting point of the film.